THE SIR SUPPLIED DR. BRITT WITH 300MG OF L-[P-15N] HISTIDINE & 300MG L-15N HISTIDINE. THE MOLECULAR OXYGEN IN OUR ATMOSPHERE IS A PRODUCT OF A ``WATER-SPLITTING'' REACTION THAT OCCURS IN THE OXYGEN-EVOLVING COMPLEX (OEC) OF THE PHOTOSYSTEM II COMPONENT OF PLANT PHOTOSYNTHESIS. THE CATALYTIC CORE OF THE OEC IS AN ENSEMBLE OF FOUR MANGANESEATOMS ARRANGED IN A CLUSTER OF UNDETERMINED STRUCTURE. THIS PROPOSAL CONCERNS THE USE OF NOVEL PULSED ELECTRONPARAMAGNETIC RESONANCE (EPR) METHODS DESIGNED TO INVESTIGATE THIS IMPORTANT ENZYME SYSTEM TO A DEGREE NOT POSSIBLE WITH CONVENTIONAL CONTINUOUS-WAVE EPR SPECTROSCOPY. THE GENERAL GOAL IS TO UTILIZE THE PULSED EPR TECHNIQUES OF ELECTRON SPIN ECHO ENVELOPE MODULATION (ESEEM) AND ELECTRON SPIN ECHOELECTRON-NUCLEAR DOUBLE RESONANCE (ESE-ENDOR) TO MEASURE THE NUCLEAR SPIN TRANSITIONS OF PARAMAGNETIC NUCLEI IN PROTEIN LIGANDS TO THE CLUSTER. IN PARTICULAR, WE WILL USE HISTIDINE SPECIFICALLY LABELLED WITH THE 15N STABLE ISOTOPE TO DESCRIBE LIGATION AND FUNCTION OF HISTIDINE IN THE CHEMISTRY OF PHOTOSYNTHETIC OXYGEN EVOLUTION.